It is conventional practice, in the drilling and completion of wells, to case an open hole by cementing tubular casing in place in a wellbore. Thus, the open hole is prevented from caving in, fragile formations are protected, inter-zonal communication is restricted and contamination of groundwater is prevented. In the course of cementing the casing, components are placed in the well which can later migrate and possibly interfere with well operations. To understand the phenomena, cementing operations are reviewed herein.
A string of casing is made up and lowered into the open wellbore. Prior to the placement of cement, the casing and hole are filled with drilling mud, which must be displaced for placing cement.
In the case of surface and intermediate casing, in order to reduce contamination of the interface between the displaced mud and the cement, a bottom cementing plug is placed in the casing and pumped ahead of the cement slurry. The bottom plug is typically constructed with a one piece hollow metallic or a one piece non-metallic core having an elastomer covering molded to the core. The elastomer cover typically incorporates a plurality of wipers. The function of the wipers is to wipe the internal surface of the casing, maintain the separation of fluids during the displacement of the cement slurry down the casing and provide a means of sealing upon displacement of the plug. The bottom plug incorporates a rupture diaphragm or valve that will rupture or open upon the bottom plug reaching or resting on a float shoe, float collar or landing collar located near or at the bottom of the casing. An increase in fluid pressure above the supported bottom plug results in the diaphragm rupturing, allowing the cement slurry to pass though the bottom plug and continue out the bottom of the casing, beginning to fill the annular space between the casing and the well bore.
When the necessary volume of cement has been placed into the casing, a top plug is positioned on top of the cement for separating the cement from a driving slug of mud. The top plug is typically constructed having a solid elastomer, one piece metallic or one piece non-metallic core having an elastomer covering molded to the core, the elastomer cover incorporating a plurality of wipers. Optionally, the top plug may also have a rupture element, as described in U.S. Pat. No. 5,191,932 and incorporated herein by reference in its entirety, so that if the top and bottom plugs are inadvertently reversed, in operation, cementing can continue without removal of the plug or removal of cement placed into the wellbore before the error was discovered. Pressures required to rupture the diaphragm are such that the diaphragm will not rupture during normal operations. The function of the wipers is to wipe the internal surface of the casing, maintain the separation of fluid during the displacement of cement slurry down the casing using drilling fluid and to provide a method of providing a sealing mechanism across the casing upon landing the top plug on top of the bottom plug. When displacement of the cement slurry is complete the top plug will land on top of the bottom plug and is expected to remain in this position once the cement hardens.
After the cement slurry has become hard, the top and bottom plugs are drilled out. Additional drilling can then proceed through the cemented casing. Additional lengths of casing are hung in the cemented casing and the cementing operation is repeated.
The last segment of casing to be positioned in the wellbore is the production casing. It is typically smaller in diameter than either the surface or intermediate casing and extends to the bottom of the wellbore. As no further drilling will occur after the production tubing has been run in and cemented, the plugs are not drilled out, but instead are left cemented into the bottom of the hole. As with the previous cementing operations, a bottom plug is run ahead of the cement and a top plug is run behind. Once the top plug rests on the bottom plug, pressure sufficient to keep the plugs at the bottom of the hole, but not to rupture the diaphragm in the top plug, if present, is maintained on the plugs for approximately 8 hours to permit the cement to properly set.
Once the wellbore has been cased, the casing is perforated above the plugs at a zone of interest and the wellbore is ready for production. A tubing string and pump are lowered into the casing and fluids are produced up the tubing string to surface.
Applicant is aware that in many cases, often a year or more after the cementing of the casing, the top cementing plug can migrate up the production casing to the pump intake and cause fouling of the pump. Typically, most wellbores have a minimum overhole, that is to say that the bottom of the casing is not far below the zone to be perforated. The Applicants believe that during perforation of the casing, the cement surrounding the plugs and outside the casing may be fractured. If sufficient fracturing occurs, the plugs are no longer held securely inside the casing and can migrate upwards. It is also possible that gas from the formation can travel downward through the fractured cement outside the casing and rise at the bottom of the casing to apply pressure on the plugs. If the one-way valves in the float equipment are also damaged as a result of pressure pulses during perforation, the plugs may be forced upwards due to the increased pressure from below.
Traditionally, whenever the pump intakes are fouled, production is lost and the tubing is tripped out of the well to repair the pump, at great expense. A solution that has been employed to prevent plugs from migrating upwards into the pump intake is to run a bridge plug into the casing and set it down on the top cementing plug to anchor the plug in position. Whether repairing the pump or setting a bridge plug, significant expense is involved in both equipment and rig time.
Regardless of the reason or hypothesis for plug migration, clearly there is a need for means to prevent the cementing plug from migrating up the casing. Ideally, such means would be incorporated directly into the plug, thus realizing significant cost and time savings.